Tropical tree cover will jump sharply between a forested state and savanna or treeless conditions rather than respond smoothly to climate change, according to a study by scientists from Wageningen University published in the journal Science.
The study shows that the resilience of forest and savanna is related to rainfall in an almost universal way on the three continents studied, Africa, Australia and South America. The authors used the resulting empirical model to predict where existing forest is most fragile, but also to reveal where a treeless situation may be most easily tipped into a savanna, or where savanna may be turned into forest with little effort.
"Tree cover is one of the most defining aspects of landscapes," says Milena Holmgren, one of the authors and a specialist on plant ecology. "It is therefore remarkable how poorly we understand what determines where we have forest, and where savanna or desert. Obviously rain is important. Deserts are found in the driest places and rainforest in the wettest. However, what happens in between has been very puzzling for scientists." The study based on massive satellite data now reveals an intriguing global pattern. Instead of a gradual increase in tree cover with rainfall, there appear to be 'forbidden' states around 5% and around 60% tree cover. Thus the system jumps between three contrasting alternative states: forest, savanna (with roughly 20% tree cover) and a treeless state.
"This is one of the most convincing lines of evidence for the existence of large scale alternative stable states in nature," says Marten Scheffer, who leads the research program on tipping points. "We were surprised ourselves how well the data supported this influential, but radical theory." While the proof of the existence of tipping points is of great scientific importance, the practical power of this study is that it also uses the data to reveal where on the globe we have the greatest risks of collapse, and where the greatest opportunities for recovery of forest. "As the system approaches a tipping point, it becomes increasingly fragile, in the sense that a small perturbation, such as a dry year or some small scale deforestation, may invoke a critical transition to the other state," says Scheffer.
"Understanding potential impacts of climate change on the Amazon forest is one of the major challenges for scientists in the region today," explains author Marina Hirota who came to work with the Wageningen team after her studies in Brazil to analyze forest resilience. "Our study now shows that the forest is most fragile precisely in the areas where pressure from human activities is also the highest. This kind of information should allow governments to make better decisions, as it shows the risks and opportunities that are inherent to the stability properties of these ecosystems that still cover massive parts of the Earth."
- Marina Hirota, Milena Holmgren, Egbert H. Van Nes, Marten Scheffer. Global Resilience of Tropical Forest and Savanna to Critical Transitions. Science, 2011; 334 (6053): 232-235 DOI: 10.1126/science.1210657
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